The present disclosure generally relates to a process for lowering CO2 emissions in a power plant utilizing fossil fuels for power generation.
Carbon dioxide (CO2) emissions from power plants utilizing fossil fuels are increasingly penalized by national and international regulations, such as the Kyoto protocol, and the EU Emission Trading Scheme. With increasing cost of emitting CO2, CO2 emission reduction is important for economical power generation. Today's CO2 removal technologies concentrate on CO2 clean up of the atmospheric flue gas stream of a power plant, which results in very large, costly and energy intensive CO2 removal units.
Gas turbine plants operate on the Brayton cycle, which generally uses a compressor to compress the inlet air upstream of a combustion chamber. Then the fuel is introduced and ignited to produce a high temperature, high-pressure gas that enters and expands through the turbine section. The turbine section powers both the generator and compressor. Combustion turbines are also able to burn a wide range of liquid and gaseous fuels from crude oil to natural gas.
There are three generally recognized ways currently employed for reducing CO2 emissions from such power stations. The first method is to capture CO2 after combustion with air from the exhaust gas; wherein the CO2 produced during the combustion is removed from the exhaust gases by an absorption process, adsorption process, membranes, diaphragms, cryogenic processes or combinations thereof. This method, commonly referred to as post-combustion capture, usually focuses on reducing CO2 emissions from the atmospheric exhaust gas of a power station. A second method includes reducing the carbon content of the fuel. In this method, the fuel is first converted into H2 and CO2 prior to combustion. Thus, it becomes possible to capture the carbon content of the fuel before entry into the gas turbine and the formation of CO2 is hence avoided. A third method includes an oxy-fuel process. In this method, pure oxygen is used as the oxidant as opposed to air, thereby resulting in a flue gas consisting of carbon dioxide and water.
The main disadvantage of the post-combustion CO2 capture processes is that the CO2 partial pressure is very low on account of the low CO2 concentration in the flue gas (typically 3-4% by volume for natural gas fired power plants) and therefore large and expensive devices are needed for removing the CO2. Although the CO2 concentration at the stack and thus the partial pressure could be increased by partial recirculation of the flue gas to the compressor of the gas turbine (in this respect see, for example, U.S. Pat. No. 5,832,712), it still remains fairly low (about 6-10% by volume). The low CO2 partial pressures and large gas volumes implicit with the form of post-combustion capture leads to very high energy costs related to CO2 removal in addition to very bulky and costly equipment. Both these factors significantly increase the cost of electricity generation.
Accordingly, there is a need for improved processes for efficiently removing CO2 from the power plant.